Sub-calibre, fin-stabilized, high-velocity projectiles are primarily used in tank guns for countering hostile tanks and their armour penetration capability depends on the velocity of the projectile when it strikes the target. Since the velocity of the projectile at the target in turn varies as a function of its muzzle velocity as it leaves the barrel from which it was fired, the highest possible muzzle velocity must be imparted to the projectile as it leaves the barrel.
This in turn requires propellant charges having a very high energy content, which at the same time, however, must have characteristics such that they are burned in the barrel during the passage of the projectile through the barrel without in the process giving rise to a barrel pressure that exceeds the maximum admissible values for the barrel.
The factor limiting the quantities of propellant and hence indirectly also the amounts of energy that can be used to accelerate the projectile in an existing barrelled weapon is usually the volume of the charge space available for the propellant charge in the barrel. Older tank guns have their respective charge spaces adapted to the propellant geometries that were available when the guns were designed, and the optimum quantity of propellant from a ballistic standpoint was then often incorporated into the design. Feasible ways of increasing the performance of these older guns might concentrate on the use of a more high-energy propellant, thereby increasing the energy content in the available charge space, which has already been done, or on increasing the density of the propellant charge, that is to say its energy content per unit volume, or both of these. The latter approach, however, must not be done in such a way that the combustion of the propellant charge is disrupted to the extent that it is no longer optimal from a purely ballistic standpoint. A propellant charge must not be more compact than the progressivity of the propellant will allow.
Until quite recently propellant charges for large-Calibre barrelled weapons such as tank guns and other artillery guns have generally consisted of freely disposed propellant grains of limited size, which may have been formed as granulated stick propellant with one or more longitudinal ignition or combustion channels, although it has also been possible to encounter propellant charges comprising a very large number of longer propellant sticks, each provided with a large number of transverse perforations, which when fired along their internal combustion channels are split up at the perforations by the gas pressure inside the combustion channels into shorter pieces, which are then burned in a manner similar to that of the granulated tubular propellant. Both of these propellant charge types, however, contain large volumes of empty space between the propellant grains or the propellant sticks.
In recent years, however, a start has been made on at least the experimental phase of updating an older idea for producing so-called multi-perforated propellant. This type of propellant is composed of propellant in block, stick or sheet form, provided with a very large number of parallel perforation channels, the internal spacing of which is intended to equal twice the distance that a propellant of the relevant chemical composition will burn during the dynamic pressure sequence in the barrelled weapon for which the propellant charge in question is intended during the period of time that a projectile launched by the propellant charge will spend in the barrel after the propellant charge has been ignited. Persons skilled in the art refer to said distance between two such combustion channels as the e-measurement of the propellant. The intention of the multi-perforated propellant is therefore that it should be ignited in all perforations and that the e-measurement should be selected so that as far as possible all the propellant will be burned before the projectile reaches the muzzle of the barrel.
The multi-perforated propellant charges are not all that easy to produce, since the e-measurement for propellants of a modern chemical composition will be from 0.5 mm up to almost 4 mm, whilst the perforation channels ought preferably to have a diameter of 0.3 to 1 mm.
As a theoretical idea, multi-perforated stick propellant is therefore by no means a novelty, even though the product in question has only recently become available to limited extent on the market. Examples of some older patents that describe the basic principles behind the multi-perforated propellant, without giving any more precise information on suitable perforation diameters and perforation intervals include U.S. Pat. No. 677,527 and GB 16,861 dating from 1895. Even in the 1890's therefore, some far-sighted engineers reasoning quite theoretically seem to have realized the advantages of the multi-perforated propellant. On the other hand we have not succeeded in finding any evidence of this having been put into practical application.
A suitable method and device for producing multi-perforated stick propellant is described in our own Swedish patent SE-518 867 (the equivalent of which is WO-02/083602). A general property of the multi-perforated propellant is that it burns progressively and hence it is possible to produce propellant charges that are very compact and thereby assume high charge weights and large energy contents per unit volume.
The sub-calibre, armour-piercing, high-velocity projectiles generally have a slender arrow shape and they are thereby relatively long, so that in cartridge form quite a large proportion of their length will protrude into the cartridge case and will thereby limit the space in the case which would otherwise have been available for propellant. Furthermore, because they are fired without any inherent spin, for control on their trajectory they are dependent upon fixed, rear-mounted stabilizing fins, which further limit and divide the space available in the cases into multiple smaller sections. Where the propellant charges consist of loose, finely granulated propellant the latter fact does not present any great problem, but as soon as one wishes to use multi-perforated stick propellant or other propellant that occurs in larger pieces, the division of the available space may present certain problems, especially when seeking to achieve extremely high charge weights where propellant charges containing unutilized vacant spaces here and there are consequently unacceptable.
In purely general terms, the availability of the multi-perforated propellant affords a fresh opportunity for producing propellant charges with extremely high charge weights, but it is then a matter of utilizing all available space for the propellant, unconstrained by the fact that it occurs in larger units such as stick, sheet or tubular form.